Audio preamplifiers are important components in sound recording, reproduction, or audio for live concerts or events. In general, an audio preamplifier takes an electrical signal generated from a microphone or another sound source as an input, and further processes and amplifies this input signal to generate a desirable level of amplified electrical signal to other components such as main amplifiers, speakers, or recording equipment.
Preamplifiers take an important role in determining amplified and/or reproduced sound characteristics of the sound source, because it is generally the first actively-powered stage for the electrical signal generated from the microphone or another sound source, which are highly vulnerable to undesirable distortions or noise introduced during any amplification stages. For example, an undesirable introduction of distortions or noise at or before the preamplifier stage may be magnified by subsequent amplification stages, thereby making post-preamplifier stage correction difficult and exacerbating any problems from the preamplifier to recording equipment or a listener.
In audio industry, impedance matching or bridging between a microphone and a preamplifier has been an important requirement for high fidelity electrical signal transmission between an output from the microphone and an input to the preamplifier. In general, the output from the microphone is an electrical signal which typically undergoes signal transformation through a transformer unit inside the microphone circuitry. It is desirable to have the impedance of this output terminal from the microphone match or appropriately bridge the impedance of the preamplifier for high fidelity electrical signal transmission between the microphone and the preamplifier. For example, the resistive impedance matching may involve keeping the impedance load value to be 3-10 times the value of a passive microphone's output transformer. In case of transformer-coupled preamplifiers, it may be desirable to match the actual impedance values (e.g. 150 ohm-output from a passive microphone's transformer to 150 ohm-input of a preamplifier's input transformer).
Conventional methods of impedance matching or bridging between the microphone and the preamplifier include using a commonly-used impedance value at the output of the microphone and the input of the preamplifier. A less used but another conventional method of impedance matching or bridging between the microphone and the preamplifier is varying the output impedance value of a passive microphone by adjusting the microphone's passive circuitry before the output of the passive microphone is transmitted to any active power elements or a preamplifier.
These conventional methods of impedance matching or bridging are typically only used for efficient signal transmission between the microphone and the preamplifier, and are not designed to produce customized effects for various sound characteristics at a preamplifier stage. Because the preamplifier is generally the first stage for active circuit processing of a sound source signal (e.g. a passive microphone electrical signal), certain customized effects for sound characteristics (e.g. an emphasis on a mid-range audible frequency, an emphasis on treble or bass, or other intended sound effects) may be best obtained at the preamplifier stage without causing significant amount of undesirable noise or distortion to the sound source signal.
Therefore, it may be beneficial to provide a preamplifier unit that can vary impedance loading conditions to obtain customized, desired, and/or adjustable sound characteristics at the preamplifier stage. It may be also beneficial to provide various embodiments of preamplifiers with different impedance loading adjustment interfaces, which may be embodied in a standalone preamplifier unit or an integrated preamplifier unit in an active microphone casing.